Liquid crystal alignment layers, which are intended to align liquid crystal molecules, are important for maintaining an orderly arrangement of liquid crystal molecules and thereby achieving optical characteristics based on the refractive index anisotropy of the liquid crystal molecules, thus serving as essential components of liquid crystal display devices. Since the display characteristics of liquid crystal display devices are greatly affected by the alignment of liquid crystal molecules, research has been conducted on various modes of alignment, which can generally be divided into two types: vertical alignment and horizontal alignment.
Liquid crystal display devices including vertically aligned liquid crystal layers (also referred to as “VA mode liquid crystal display device”) are widely used in displays because of their superior display characteristics, including high contrast. These liquid crystal display devices, however, have insufficient viewing-angle characteristics, and various techniques for improving the viewing-angle characteristics have been researched. One common technique for improving the viewing-angle characteristics is multi-domain vertical alignment (MVA), which divides each pixel into a plurality of liquid crystal domains with different orientations (i.e., forms a multi-domain structure).
To form a multi-domain structure for MVA, it is necessary to control the tilt orientation of liquid crystal molecules. One technique that has been used is to provide slits (openings) or ribs (protrusions) on electrodes. However, the use of slits or ribs have several problems. Unlike alignment layers used for tilt orientation control in the conventional twisted nematic (TN) mode, slits and ribs are linear and may thus have uneven anchoring force on liquid crystal molecules within the pixels. This results in a distribution of response rates. Slits and ribs also decrease the light transmittance of the regions where they are provided. This results in decreased display brightness.
Another technique for controlling the tilt orientation is polymer-sustained alignment (PSA) technology, which fixes the tilt orientation of liquid crystal molecules by polymerizing a photopolymerizable or thermally polymerizable monomer added to the liquid crystal in advance while applying a voltage to tilt the liquid crystal molecules (see PTL 1). This technique could solve the problems associated with the use of slits or ribs, i.e., a distribution of response rates and decreased light transmittance. This technique, however, has other problems, including variations in characteristics due to the monomer added to the liquid crystal, difficult process control, and the influence of residual monomer.
It is therefore preferred for VA mode liquid crystal display devices to have a multi-domain structure formed by tilt orientation control using alignment layers. One technique for inducing the force for controlling the tilt orientation is rubbing, which involves forming a layer such as a polyimide layer on a substrate and rubbing the layer with rubbing cloth to control the orientation and pretilt angle thereof. Rubbing, however, is not suitable for forming a fine mufti-domain structure and also has a problem in that it generates triboelectric charge and impurities.
Examples of liquid crystal display devices including horizontally aligned liquid crystal layers include IPS mode liquid crystal display devices. IPS mode liquid crystal display devices are widely used in displays because of their superior display characteristics, including low viewing angle dependence of contrast and color. IPS mode, however, requires the pretilt angle to be controlled to 1° or less on the electrode surfaces to reduce the viewing angle dependence and color reproductivity in the black state.
Rubbing is also commonly used as a technique for inducing the force for controlling the tilt orientation to achieve horizontal alignment. However, one problem arises in that the process of rubbing a polyimide layer to achieve horizontal alignment induces a pretilt angle of more than 1° to liquid crystal molecules, which makes it difficult to achieve the desired display characteristics.
As discussed above, the control of the orientation and pretilt angle of liquid crystal molecules using alignment layers is important for improving the display characteristics fox both vertical alignment and horizontal alignment.
Besides rubbing, photoalignment is known as a technique for inducing the force for controlling the tilt orientation (see PTL 2). Photoalignment facilitates the formation of fine multi-domain structures with varying light exposure patterns and also involves no contact with the layer and thus generates little or no triboeleetric charge and impurities. Photoalignment would therefore solve the foregoing problems and improve the display characteristics.
Examples of known materials that can be used to achieve alignment by photoalignment include compounds having photochemically isomerizable sites, such as azobenzene derivatives (see PTL 3); compounds having photochemically crosslinkable sites, such as cinnamic acid derivatives, coumarin derivatives, and chalcone derivatives (see PTLs 4, 5, and 6), and anisotropically photodegradable compounds such as polyimide derivatives.